1. Field of the Invention
The present invention relates to an image processing apparatus for applying optimal image processing in accordance with image features included in an original image, an image processing method, a program, and a recording medium.
2. Description of the Related Art
As an image processing apparatus for applying optimal image processing according to image features, an apparatus which extracts a character edge using a primary or secondary derivative to classify an image into a character area and a remaining (i.e., non-character) area, and then applies adaptive processing is known.
Also, an apparatus which processes the edge of a character drawn in a halftone dot area to look sharper, when the pixel to be processed is located within a halftone dot area and neighbors a pixel that forms the edge of a character or the like, is known. For example, see patent reference 1: Japanese Patent Laid-Open No. 2002-281313.
The aforementioned technique reduces the weight of smoothing processing for a specific part in an area to be smoothed. However, since the weight of the specific part is reduced, smoothing arithmetic operations become complicated.
In general, the smoothing processing makes a convolution operation on pixel values in the area to be smoothed with weighting coefficients, and divides the result of the convolution operation by the sum of the weighting coefficients. At this time, when a specific weighting coefficient is reduced, the division coefficient is changed, resulting in a complicated arrangement upon hardware implementation. When a weighting coefficient that does not change the division coefficient is used, its value is limited, and the same effect cannot be expected. Furthermore, when a change in division coefficient is ignored, the desired effect cannot be expected, and image quality may deteriorate. In addition, when change is performed so as to reduce the weight, the processing cannot provide a gain of desired space frequency response, and the level of smoothing becomes insufficient. Further, when a part of the weight is set to “0”, isotropy of filter processing is lost and the effective condition of filtering is different depending on the direction. As a result, image quality lacking in uniformity might occur.
As another issue, upon switching filters between a character part and other parts (photo part, halftone part, background part, and the like) on a document with a white background, it is desired to apply emphasis processing to the character part and smoothing processing to other parts. When this is done, however, a contour may be generated around the character part. This is because the character emphasis processing is applied to the character part, but a background part around the character part is determined as a part other than the character part (i.e., a non-character part) and undergoes the smoothing processing. When the smoothing filter size is large, signals of the character part also undergo smoothing, and a contour is generated around the characters.
The aforementioned method is one of the solutions, but it suffers from the aforementioned problems which remain unsolved.